Impact of Optical Purity on Light Harvesting Property in

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Spectroscopy and Photochemistry; General Theory

Impact of Optical Purity on Light Harvesting Property in Supramolecular Nanofibers Ramarani Sethy, Rémi Métivier, Arnaud Brosseau, Tsuyoshi Kawai, and Takuya Nakashima J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02015 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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The Journal of Physical Chemistry Letters

Impact of Optical Purity on Light Harvesting Property in Supramolecular Nanofibers Ramarani Sethy†, Rémi Métivier,*,‡, Arnaud Brosseau,‡ Tsuyoshi Kawai,*,† and Takuya Nakashima*,† †

Graduate School of Science and Technology, Division of Materials Science, Nara Institute of

Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan ‡

PPSM, ENS Cachan, CNRS, Université Paris-Saclay, 94235 Cachan, France

AUTHOR INFORMATION Corresponding Authors *R. M.: E-mail: [email protected] *T. K.: E-mail: [email protected] *T. N.: E-mail: [email protected]

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The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

Supramolecular ordering and orientation of chromophores are tremendously accomplished in the photosynthetic light harvesting complexes, which are crucial for long-range transfer of collected solar energy. We herein demonstrate the importance of optical purity on the organization of chromophoric chiral molecules for the efficient energy migration. Enantiomeric bichromophoric compounds, which self-assemble into nanofibers capable of chiral recognition, were mixed to form supramolecular co-assemblies with variable enantiopurity. The chiral molecules selfassembled into extended fibers regardless of enantiopurity, while their morphology was dependent on the enantiomeric excess. The optical purity of assemblies also had an effect on the emission efficiency; the nanofibers with higher enantiomeric excess afforded larger emission quantum yield. The presence of opposite enantiomer is considered to deteriorate the chiral molecular packing suitable for the directional growth of nanofiber, efficient exciton migration and chiral guest recognition.

TOC Graphic

Heteroch iral

Ch iral g u est

Hom och iral

Ch iral g u est

Poor m ig ration Efficien t m ig ration


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Biological systems have high level of structural complexity which work in a synchronized manner to exhibit efficient and rigorous functionalities. One of such examples is natural light harvesting systems in which spatial arrangements of photosynthetic pigments lead to unidirectional and efficient excitation energy flow towards the reaction center.1-5 The α-helix subunits in the light harvesting complexes are considered to arrange the chlorophyll molecules with highly accurate distance and orientation in a non-covalent manner.2,3 Attempts to mimic such light harvesting systems and the driving principles behind their mechanism have led to the development

of

nanostructured

self-assemblies

with

supramolecular

chromophoric

organization.6-16 It has often been observed that small modifications on the building units have a great effect on the organization and molecular packing in those self-assemblies, modulating the energy transfer efficiency.12 A pair of enantiomeric molecules essentially exhibit identical physicochemical properties in the molecularly isolated state while their interactions are differentiated between homochiral and racemic pairs. Chirality therefore can be one of factors affecting the ordering of molecules in such chromophoric assemblies. Recently, the importance of chirality has been recognized in some organic electronic devices17-19 such as field effect transistors20,21 and photovoltaics.22,23 The device performances such as carrier mobility and power conversion efficiency varied between the devices fabricated with homochiral and racemic molecules. The chirality of molecules impacts on the molecular packing in the devices, which has a predominant influence on the carrier transportation property. Meanwhile, the effect of enantiopurity has long been investigated on supramolecular polymerization systems. Chiral molecules capable of self-assembly sometimes form nanofibers with an appearance of onehanded helical twists, wherein the handedness of twist was determined by the chirality of building units. Previous investigations have demonstrated optical purity-dependent self-



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assembling behaviors such as continuous morphological changes in the twisting appearance,24-27 self-sorting homochiral assembly28-31 and modulation of nanofiber length.32 These behaviors are highly dependent on the molecular structure and self-assembling modes, i.e., intermolecular interactions such as hydrogen bonding and π-π stacking interactions. While the effect of optical purity on the morphology and chiroptical property including circular dichroism (CD) has been extensively discussed for supramolecular polymers, the impact on the energy harvesting and transportation properties are yet to be investigated. Recently,

we

reported

the

self-assembly

of

bichromophoric

core

substituted

naphthalenediimide (NDI) derivatives (1 in Scheme 1) into chiral supramolecular nanofibers, demonstrating an absolute enantioselective recognition of chiral perylenediimide (PDI) molecule with an opposite handedness (2 in Scheme 1).33 This chiral recognition of nanofibers towards the guest PDI molecule resulted in an efficient light harvesting behavior, in which the NDI units in the nanofibers efficiently transport the exciton energy to the bound guest molecule. However, the impact of enantiomeric compositions on the exciton migration in the NDI-based nanofiber and on the chiral recognition property have never been characterized. In the present work, we investigated the effect of optical purity of building units on the energy transporting property of supramolecular polymers prepared by mixing enantiomeric 1. The exciton energy migration along the supramolecular nanofibers was rationally evaluated by means of fluorescence and its anisotropy decay measurements. The results clearly demonstrated that the chirality of building units played a crucial role in the chromophoric ordering suitable for the efficient energy transportation as well as chiral guest recognition.


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Scheme 1. Molecular Structures of (a) (S)-Enantiomer of Nanofiber Forming Compound 1 and (b) (R)-Enantiomer of Chiral Guest Molecule 2.

We first compared the supramolecular polymerization of enantiomeric pure and racemic coassembly

of

compound

1.

The

self-assembly

was

performed

in

chloroform/methylcyclohexane(MCH) (1:19) at 1 × 10-5 M concentration upon annealing to 95 o

C followed by cooling to room temperature. Both systems afforded the nanofibers whereas the

morphological features varied between the enantiopure and racemic compositions. Enantiopure 1 formed well defined nanofibers with rather straight features while the racemic nanofibers involved more curved and winding appearance in scanning electron microscope (SEM) images (Figure 1). Given the compound 1 prefers the homochiral binding over the heterochiral one, no morphological change is supposed to be observed between the homochiral and heterochiral compositions. The change of nanofiber appearance therefore suggested the heterochiral binding is preferred in the co-assembly of 1. Some chiral self-assemblies predominantly driven by intermolecular hydrogen bonding interactions were reported to exhibit more obvious changes, such as in the degree of twisting, or morphological transitions from helical ribbons to nanotubes depending on the enantiomeric excess (ee). 24,25,27 In the present case, the decrease in the ee value resulted in an increase in the occurrence of curved and twisting feature while the width of 5 ACS Paragon Plus Environment


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nanofibers was kept to be about 6 nm for all compositions (Figure S1 in the Supporting Information).

Figure 1. SEM images of nanofibers prepared in an MCH rich solution with (a) enantiopure (S)1 and (b) racemic mixture of 1.

While compound 1 afforded nanofibers regardless of the ee composition with varying the curved and winding appearance, the emission efficiency was highly dependent on the ee value. A sigmoidal plot of emission quantum yield was obtained in response to the ee value, showing the expression of a certain degree of “majority rules” effect (Figure 2).34 The emission quantum yield (QY) of 14% was obtained for the molecularly dispersed state of 1 in chloroform while the emission property was maintained in the homochiral assemblies as QY of 20% but dropped to 9% in the racemic nanofibers. Unlike aggregation induced emission (AIE) fluorophores,35 the aggregation of conventional dyes often results in a decrease of emission efficiency to form species undesirable for emission with strong inter-molecular interactions. Since the QY for the racemic mixture in the apparent aggregate state was less than that in the molecularly dispersed state, the AIE mechanism cannot well explain the higher QY for the homochiral nanofibers. The


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rather enhanced emission efficiency in the homochiral system could be attributed to the contribution of efficient energy transportation to emitting sites,36 while the less efficient excitonic energy migration property in the racemic fibers most likely led to the significant decrease of emission QY.

0.2

Emission quantum yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.18 0.16 0.14 0.12 0.1 0.08

0

0.2 0.4 0.6 0.8 Enantiomeric excess of (R)-1

1

Figure 2. Emission quantum yield of 1 in MCH-rich solvent with varying ee values.

Time-resolved fluorescence decay analysis on compound 1 in chloroform and MCH rich solutions with varying the ee value was performed upon excitation at 440 nm. The distinct decay profiles of monomer and aggregates with different ee values were monitored at 500 nm emission region, giving the clear indication of emission from different species (Figure 3a). The chloroform solution in the molecularly dispersed state exhibited mono-exponential decay with an average lifetime of 1.64 ns supporting the presence of a unique emissive species. All the fluorescence decays of assemblies in the MCH-rich solvent were fitted together in a four exponential function with χ2 values of 1.0–1.2, consisting of three major lifetime components and a negligible one 7 ACS Paragon Plus Environment


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(Table S1, Figure S2). As an example, the decay of enantiopure (R)-1 exhibited lifetimes of 3.81 ns (τ1, 58%), 1.47 ns (τ2, 25%), 0.17 ns (τ3, 16%) and 9.36 ns (τ4, 1%). The plots of preexponential factors (αi) with respect to the ee values suggested that the contributions of species with the time constants of τ1 = 3.81 (α1) and τ2 = 1.47 ns (α2) change in response to ee while those for the other components (α3, α4) are almost constant and correspond to a minor fraction of intensity (counting for less than 4% of the total collected photons for ee = 1, and less than 13% for ee = 0) (Figure 3b). Since the time constant of 1.47 ns (τ2) is close to the lifetime for the monomeric molecule (1.67 ns), it should be responsible for the monomeric units present in the assemblies. Meanwhile, the major lifetime component of 3.81 ns (τ1) should be ascribed to an excited species delocalized in the assembly with the multiple NDI-stacks. The increase in α1 for the long-lifetime species clearly accompanies the decrease in α2 for the monomeric one on increasing the ee value. This result suggests the enhanced delocalization of excited state in the more enantiopure assemblies. It is noteworthy that the slight decrease in the ee value from 1.0 to 0.8 led to the marked decrease in α1 and the concomitant increase in α2 (Figure 3b), suggesting that the presence of heterochiral binding sites have a significant effect in inhibiting the exciton delocalization property. Such the marked effect was not observed in the ee-dependent emission QY (Figure 2). While the emission QY thus appears to have a certain relationship with the relative contribution of long-lived emissive species with the enhanced exciton delocalization property, an increase in the non-emissive sites with decreasing ee should be mostly responsible for the decrease in QY below ee of 0.6.


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Figure 3. (a) Time-resolved fluorescent decay (at 500 nm) profiles of (R)-1 in chloroform and 1 (with varied ee) in MCH-rich solvent together with an instrumental response function (IRF). (b) Plots of pre-exponential factors as a function of ee. To prove that the exciton energy migration along the nanofiber has a predominant effect on the long-lived emission species, we performed time-resolved fluorescence anisotropy measurement (TRAM).37 The time-resolved anisotropy of 1 in chloroform exhibited an initial anisotropy of 0.19 which depolarizes to zero with a decay time of 0.35 ns (Figure 4, Figure S3, Table S2). The relatively long time constant for the anisotropy memory loss could be attributed to molecular rotational diffusions.37 The enantiopure assemblies of (R)-1 gave an extremely fast decay constant (τr) of < 20 ps (below the timescale of instrumental acquisition range) for depolarization with a residual anisotropy (r∞) of 0.065. This very fast depolarization decay 9 ACS Paragon Plus Environment


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constant is considered to originate from the fast exciton migration between the NDI units in the nanofibers, leading to an exciton migration constant (kem) over 5 × 1010 s-1.38-40 The residual anisotropy value (r∞) of 0.065 may be attributed to the inherent orientation of NDI units in the nanofibers with suppressed diffusivity.37 The depolarization in fluorescence became slower by decreasing the enantiopurity in the nanofibers. A slight decrease in the enantiopurity significantly interrupted the exciton migration as suggested by the marked elongation of τr (Table S2), which is consistent with the result of emission lifetime study (Figure 3). The anisotropy decay time constants showed a clear relationship with the ee values (Figure S3, Table S2). This result supports that the rapid and efficient exciton migration takes place in the homochiral nanofibers along with substantial loss of exciton polarization, while the heterochiral assemblies display considerably fixed excitons in space and in polarization. The residual anisotropy value (r∞) is also dependent on the ee value; the higher ee gave the larger r∞ values, suggesting the optical purity has an effect on the orientation of NDI units in the nanofibers. The non-zero r∞ value even for the racemic assembly indicated the presence of a certain degree of orientation in the NDI assemblies as discussed below.


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Figure 4. Time resolved fluorescent anisotropy measurements of 1 with varied ee values together with that of (R)-1 in chloroform monitored at emission maxima 500nm wavelength. The structure of nanofibers with varied ee was characterized by means of absorption, Fourier transform infrared (FTIR) and X-ray diffraction (XRD) measurements to evaluate the effect of optical purity on the molecular packing. Two characteristic absorption bands were observed at around 450 and 340 nm in the UV-vis spectra (Figure S4). The former was assigned to the intramolecular charge transfer (CT) between the electron donating ethoxy group and the electronic withdrawing carbonyl group in the core-substituted NDI.41 The electronic transition along the NDI long axis is responsible for the latter one at 340nm, which is sensitive to the close inter-NDI interactions through π-π interaction.33,41-43 This NDI-band exhibited a continuous blue-shift with a decrease in the 0-0 band and an increase in the higher vibronic bands (0-2 and 0-3) upon cooling from 90 to 25 oC (Figure S5), corresponding to the self-assembly of 1 with inter-NDI-stacks.33,43 The shape of CT band is almost independent of the ee value. Interestingly, the decrease in ee resulted in the sharpening with a clear emergence of vibrational structure in the NDI-band at the shorter wavelength. The absorption spectral change thus suggested the stronger inter-chromophoric interactions in the nanofibers with the higher ee value and the more suppressed inter-NDI interactions even in the self-assembled nanofibers for the racemic mixture as discussed in more detail later. FTIR measurements on self-assembled states at varying ee values exhibited three characteristic bands at around 1700, 1650 and 1630 cm-1 corresponds to symmetric (νs) and asymmetric (νas) stretching of imide-carbonyl groups and amide I (C=O stretching), respectively (Figure S6).43 The gradual shift in the amide I band from 1632 to 1634 and 1635 cm-1 for the nanofibers with ee of 1, 0.6 and 0, respectively, indicating a slight modification in the hydrogen11 ACS Paragon Plus Environment


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bonding mode depending on the ee value. While the minor modification was observed for νs of imide-carbonyl at around 1655 cm-1, almost no shift was found at 1705 cm-1 for the νas with varying ee values. The corresponding imide-carbonyl groups seems thus not to be involved in the hydrogen bonding network. Powder XRD (PXRD) patterns of compound 1 in the selfassembled states deposited from the MCH-rich solutions with different ee values were recorded (Figure S7). All samples possess a peak at around 4.5º corresponding to the d-spacing of about 20 Å. This peak corresponds to the length of packing unit and a slight shift to the small angle direction with decreasing ee suggests the change in the packing structure in the assemblies. Furthermore, the peaks at 2.5º, which could be assigned to the cylindrical assemblies with a rotational displacement of cyclohexane-diamide units due to the chirality,44,45 were observed only for non-racemic assemblies (ee = 1, 0.4). The corresponding peak seems to appear below 2º for the racemic assembly, which is below the measurement range for the PXRD study. The results in UV-vis absorption, FTIR and PXRD studies thus clearly suggested that the molecular packing structure dominating the inter-chromophoric interaction was modulated in the selfassemblies in response to the optical purity of building units. The packing structures in homochiral and racemic assemblies are simulated with a molecular mechanics study using a COMPASS force field46 by a Materials Studio 7.0 program. The intermolecular hydrogen bonding network between the amide groups was successfully formed in a parallel packing structure for the homochiral assembly while an anti-parallel packing mode successfully affords the hydrogen bonding network for the racemic assembly (Scheme 2 and Figure S8),47 corresponding to the shift in the amide I band (Figure S6). The antiparallel assembly for the heterochiral binding with the less steric crowding should be preferred in the racemic assembly. Taking these packing structures into account, the difference in the energy 12 ACS Paragon Plus Environment

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migration efficiency and the nanofiber morphology can be well explained. The diethoxysubstituted NDI units are arranged closely in the homochiral parallel packing structure in favor of the efficient energy migration. The highly regular molecular arrangement with the rotational displacement dictated by the central-cyclohexane-diamide chirality should afford the directional growth of nanofibers for the homochiral assembly. Meanwhile, the heterochiral stacking arranges NDI units oppositely adversely operating for the excition delocalization. It should be noted that the practical racemic assembly contains both the heterochiral and homochiral binding in a random manner and such the stressed irregular assembly is responsible for the appearance of curved and winding fiber morphology in the SEM image (Figure 1b) as well as the less NDIorientation suggested by the r∞ value.



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Scheme 2. Simulated Packing Structures (top) together with Schematic Illustrations (bottom) for (a) Homochiral (S-1) and (b) Heterochiral Assemblies (Tetramers). (H atoms and alkyl-tails are omitted in the simulated structures for clarity).

Finally, the effect of enantiopurity on the light harvesting property was investigated. (S)-2 compound was added to the nanofiber solutions of 1 composed of both enantiomers with varying ee. The excitation energy was transferred to the PDI units in the guest molecule through the energy migration between NDI units in the nanofiber upon recognition, leading to the sensitized emission of (S)-2.33 The emission profiles of 1 with varying ee from –1 ((S)-1) to 1 ((R)-1) changed upon the addition of 1.5 mol% of (S)-2 in the MCH-rich solvent (Figure S8). The energy harvesting property was evaluated on the basis of relative emission intensity at 480 and 14 ACS Paragon Plus Environment

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540 nm (I540/I480), corresponding to the emission from NDI and PDI units, respectively (Figure 5).33 The I540/I480 value almost remained constant below ee 0.2, indicating the lack of sensitized emission of (S)-2. The energy migration property is expected to be enhanced with increasing the enantiopurity of (S)-1 toward ee of –1 whereas the absence of sensitized emission of (S)-2 clearly supports the absolute enantioselective recognition capability of 1 for the guest 2 with the opposite handedness.33 The I540/ I480 value showed an increment upon increasing ee over 0.2. Although the similar emissivity of the donor assemblies was suggested between the ee values of 1 and 0.8 (Figure 2), an apparent difference was observed in the sensitization capability of acceptor emission in this ee-range (Figure 5). Given the strong inhibition of exciton migration by the heterochiral binding sites as demonstrated in the fluorescence and its anisotropy decay measurements, the observed ee-dependent energy transfer property suggests that the exciton migration among the donor molecules most likely plays a definitive role in the light harvesting processes at the high ee-range. While the presence of heterochiral binding sites of 1 should also have an effect on the chiral recognition of 2, it cannot be evaluated independently through the energy transfer involving the exciton migration process among the donor molecules.



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Figure 5. Relative emission intensity at 480 and 540 nm (I540/I480) with varying the ee values of (R)-1 in presence of 1.5 mol% of (S)-2 in MCH-rich solvent. Concentration: [(R)-1 + (S)-1] =1× 10-5M, [(S)-2] =1.5×10-7M.

We have demonstrated the impact of the enantiopurity of self-assembly component on the light harvesting behavior including the energy migration efficiency and chiral recognition property. Upon changing enantiomeric excess we were able to manipulate the molecular ordering in the NDIs assemblies due to the heterochiral binding preference. This self-discrimination behavior resulted in the modulation of the packing structure with varied NDI arrangements, leading to the ee-dependent exciton migration behavior. The heterochiral assembly in the nonenantiopure nanofibers notably deteriorated its exciton migration capability. Although we could not directly address the effect of homochirality in nature, the present study evokes the importance of chirality in the biological functions.

ASSOCIATED CONTENT


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Supporting Information. Experimental details and supporting data including additional spectroscopic data, TEM images, and molecular modeling (file type, i.e., PDF)

AUTHOR INFORMATION Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was partly supported by JSPS KAKENHI Grant Numbers JP16H06522 (TN) and JP26107006 (TK) in Scientific Research on Innovative Areas “Coordination Asymmetry” and “Photosynergetics”, respectively. The Labex NanoSaclay (program Investissements d’Avenir, ANR-10-LABX-0035), the ENS Paris-Saclay, and the CNRS International Associate Laboratory Nano-Synergetics are also acknowledged for their support.

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